This invention relates generally to digital wireless mobile communications.
Code division multiple access (CDMA) for digital wireless mobile communications involves using correlation techniques to allow a receiver to decode one signal among many that are transmitted on the same carrier at the same time. Each user""s signal includes a unique code that appears to be noise to all except the correct receiver. A channel in the code domain describes a combination of a carrier frequency and a code. CDMA generally starts out with a narrow band signal, which for full speech is 9600 bps. This signal is spread with the use of specialized codes to a bandwidth of 1.23 Mz. The ratio of the spread data rate to the initial data rate is called the processing gain.
When transmitted, a CDMA signal experiences a high level of interference, dominated by the coded signals of other CDMA users. This interference may take two forms. Interference may arise from other users in the same cell and interference may occur from adjacent cells. The total interference also includes background noise and other spurious signals. When the signal is received, the desired signal is recovered and the interference is rejected. A correlator uses processing gain to pull the desired signal out of the noise.
Currently available cellular technology makes use of what is called second generation or xe2x80x9c2Gxe2x80x9d technology. Initially, cellular telephone technology was implemented with Advanced Mobile Phone Systems (AMPS) which were analog. In about 1995, digital systems, such as CDMA, were introduced.
A number of competing third generation or xe2x80x9c3Gxe2x80x9d technologies are being debated within the industry at this time. The goal of the 3G technologies is to offer higher bit rate services. Such services may include multimedia, including video, Internet and electronic mail.
One standard for 3G technologies is the IMT-2000 standard which was propounded by the International Telecommunications Union (ITU). IMT stands for International Mobile Telecommunications and IMT-2000 is the name for Future Public Land Mobile Telecommunications Systems (FPLMTS). FPLMTS is targeted at developing mobile telecommunications systems to be used xe2x80x9canywhere-anyplacexe2x80x9d around the year 2000 operating at approximately 2000 MHz.
In 3G technology, a xe2x80x9cbitxe2x80x9d is the fundamental information unit of input data. A xe2x80x9csymbolxe2x80x9d is a grouping of data bits based on modulation. Thus, a symbol arises after encoding but prior to spreading. A xe2x80x9cchipxe2x80x9d is the minimum bit period of the final spread data. xe2x80x9cChannelsxe2x80x9d include physical channels that are transmitted in the air, defined by a frequency and code. A transport channel is defined by how the data is sent and logical channels are defined by the type of data.
The so-called wideband or W-CDMA technology has been proposed as the 3G solution by the European Telecommunications Standards Institute (ETSI) as their proposal to the ITU for IMT-2000. ETSI""s proposal is identified as UTRA (Universal Mobile Telecommunication System Terrestrial Radio Access). (The standard can be found at www.itu.int/imt/2-radio_dev/proposals/index.html.)
Under these standards, four frequency bandwidths of 1.25 MHz, 5 MHz, 10 MHz and 20 MHz have been designated with corresponding chip rates of 1.024 Mcps, 4.096 Mcps, 8.192 Mcps and 16.384 Mcps respectively. Thus, to enable a system to handle any of the four currently specified chip rates now or in the future, four different filters may be needed for each of the frequency bandwidths or chip rates.
Thus, there is a need for a better way to enable W-CDMA cellular telephones to accommodate two or more potential chip rates.
In accordance with one aspect, a finite impulse response filter device having a selectable chip rate includes a discrete-time square-root-raised-cosine filter and a circuit. The circuits adjust the output of the filter based on the selected chip rate.
Other aspects are set forth in the accompanying detailed description and claims.